As a means of obtaining more efficient immunotherapy of tumors, we are investigating possible methods for increasing the cytotoxicity of antibodies. Antitumor antibodies have been conjugated to various toxic agents such as radioactive iodine, diptheria toxin, nitrogen mustards and enzymes with subsequent conversion of protoxins to toxins. selective cytotoxicity has been demonstrated in a number of antitumor antibody-tumor cell models in vitro. These promising systems are being tested in vivo with immunoglobulin-enzyme conjugates in several tumor models in hamsters and mice. Extensive clearance and localization studies with antibodies, antibody fragments and immunoglobulin E are also being conducted to find ways of improving selective tumor localization. Purification of antibody-enzyme conjugates is also being attempted with affinity chromatography techniques.